NEW YORK, Nov 5 (Reuters) - An index of stocks on major markets were flat on Monday, following four days of a bounce-back from a 14-month low, ahead of a key vote to define the power balance in Washington and a meeting of the U.S. monetary policy makers.

Oil prices rose in choppy trading after five days of heavy losses as the United States imposed a range of punitive sanctions on Iran, aiming to curb exports by the Islamic Republic.

Stocks on Wall Street were mostly up but the Nasdaq index was pressured by sharp declines in shares of Apple and Amazon. Nikkei reported Apple has told two smartphone assemblers to halt plans for additional production lines dedicated to the lower-cost iPhone XR.

Investors were also cautious ahead of the U.S. midterm elections. Opinion polls show a strong chance the Democratic Party could win control of the House of Representatives after two years of wielding no practical political power in Washington, with Trump's Republican Party likely to hold the Senate.

"What's spooking the market is not Congress or Senate - what's spooking the market is the volatility of Trump," said Gregory Perdon, co-chief investment officer at Arbuthnot Latham. "I'm not convinced if there's a change of control that would be able to temper that."

The Dow Jones Industrial Average rose 140.87 points, or 0.56 percent, to 25,411.7, the S&P 500 gained 8.65 points, or 0.32 percent, to 2,731.71 and the Nasdaq Composite dropped 53.73 points, or 0.73 percent, to 7,303.27.

In crude oil markets, prices rose despite a temporary U.S. exemption granted to eight countries allowing them to continue buying Iranian oil. U.S. officials have said the aim is eventually to stop all of Iran's oil exports.

U.S. crude rose 1.01 percent to $63.78 per barrel and Brent was last at $73.79, up 1.32 percent on the day.

U.S. Treasury yields fell as traders braced for $83 billion worth of government debt supply tied to the November refunding this week and awaited the outcome of the U.S. congressional elections on Tuesday.

With the Federal Reserve meeting on Wednesday and Thursday, the prospect of even tighter U.S. monetary policy after strong economic data is also on investors' minds.

Benchmark 10-year notes last rose 5/32 in price to yield 3.197 percent, from 3.214 percent late on Friday.

The 30-year bond last rose 11/32 in price to yield 3.4344 percent, from 3.454 percent late on Friday.

BREXIT HOPES BOOST STERLING

Sterling touched a two-week high against the U.S. dollar after a Sunday Times report that an all-UK customs deal will be written into the agreement governing Britain's withdrawal from the European Union on hopes of a Brexit deal, before paring gains slightly.

Prime Minister Theresa May's office said the report was speculative, but that 95 percent of the withdrawal agreement was settled and negotiations were ongoing.

Sterling was last trading at $1.3019, up 0.38 percent on the day.

The dollar index fell 0.17 percent, with the euro up 0.14 percent to $1.1401.

Market analysts warn that an unexpected outcome at the Tuesday U.S. congressional elections could undermine the greenback which has rallied more than 7 percent from April lows against its peers.

"Much of Trump’s pro-growth, pro-markets agenda has arguably been enacted in the first two years of his presidency while he’s had the backing of both the House and Senate," Craig Erlam, senior market analyst at Oanda, said.

"If the Democrats take control of the House, for example, it may inhibit his final two years which investors may not be particularly upset about given his determination to engage in a trade war with China and the EU," he said.

The Japanese yen strengthened 0.01 percent versus the greenback at 113.20 per dollar.